I posted a couple pics in Ed Turco's thread showing the small progress I've made with the Dialyte project i started months ago. Got it fairly well polished on the convex side to a smooth surface, likely well under 1/4 wave surface error. Shown, is this objective sitting on the window sill in a cobbed stand of sorts. I set it up to use at a target bolted to the side of my barn about 35 yards away. There are a couple things that should be noted about the pics- both pertaining to edges. If you look close, you'll see an edge issue on the -LEFT- of the lens, right at the bevel. None can be seen on the -RIGHT-. A good way to make a quick test for gross edges without any testing.

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Obviously, this shows the edge problem is on the concave back side of the lens, which is only pad polished. The convex front has been pitch polished for a couple hours, and no such artifact can be seen on the right; nor is it in a position to be affected by the TDE of the back side. Cool, huh?

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This last pic shows the dialyte layout. With an overcorrected achro with very little power, the longitudinal color can be corrected to a large degree, and leave residual lateral color nearer the edge of the field. If an ordinary achro is used, it "becomes a Hypo" where longitudinal color also remains. On the target on the barn, I used a piece of shelf-liner, and shot some black paint thru it onto a white background. It formed a matrix of ~1/8" dots---quick and dirty, relatively adequate for quick looks. *** With a 40mm Plossl, i estimate the mag as somewhere near 40-50x. A bit much for what John Wall recommends, but it gave me something to look at. There was some aura around the dot matrix, but not too obtrusive considering what the optical layout was. Now to the interesting part. I swapped the 40mm for a 12, and the matrix exibited a rather very sensitive depiction of the longitudinal color. The dots first focused blue, with a red background in between them. As I brought it back thru focus, they darkened up somewhat, but showed a purplish aura. Even further, and the dots changed to red with a blue background. This was extremely sensitive, and I was able to get a range from the blue dots to red dots of around 3-4mm~ (1/8"+) This effect will club you over the head! Very apparent, and very simple to do. I believe I can use it later when I get further along with development when checking the residual color issues with this design.M.

Maybe if someone has doubts about the color correction in a refractor they have, this test could be used to make a rough check. I can certainly attest it works well in this case. Here's what I used on the back end- a 70mm short focus achro- nothing particularly special; along with an eyepiece held near the diagonal's prism.

If I make a zero power/overcorrected achro or use a std achro with a negative flint, I should get spots something like this---- (probably a lot of you are thinking- 'he's lost his marbles!' but there's a couple good reasons for me persueing this. First one, is money- I'm tighter than a bulls behind???)) Here's a plot of the zero power achro version I have in mind:

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The second thing- if a positive lens is used behind the eyepiece, after this thing passes thru focus, the positions of the red-blue is -REVERSED-; (blue focuses closer, red further out) and it can be varied with some spacing change. This thing could be tuneable for the lateral color issue quite nicely. Now it's rather expensive to get hold of a good quality chunk of flint for a "normal" 2 element achro of any appreciable diameter. I don't know how many times I've requested a quote and just ignored, or blown off. Crown, schmrown- I can get a whopping piece of that in 12-16" diameter, fairly readily--- SEE WHERE THIS IS GOING??? Now who the heck would refuse a look thru a refractor of sorts @ 12-16" diameter? and he could do it on the ground, too. (a boon for us old timers, etc.) Interesting, or am I still a laffing stock (or shy a couple bricks???) One other thing- the design I have for the zero power achro, uses only 2 radii, and a heavy, short flint that grinds and polishes astonishingly easy. I once polished FULLY a 70mm diameter piece of this stuff about a year ago in 15 -MINUTES-! (SF1)M.

I get it! What this means is that you only need one piece of 8" diameter glass! You might be a cheapskate, but a smart one. Is there a specific Refractive Index that works better, or can you use any kind of glass?

The piece used in the pic, is from newport; BK7 the defacto standard, and cost about as much as Supremax stuff. I have a piece of SF1 flint that's 4.55" diameter and half inch thick that I got from Glassfab for about a C-note to be used in the correcting achro.YOU ARE RIGHT ON THE MONEY!! Only one big piece of crown glass is necessary, which should be a cinch.***********

I am not to be credited for the basic idea of this design. John Wall (CRAYFORDJON) was the originator of it, and I'm simply trying to carry it to a next step, as it were; with a little improvement that could make it a legitimate acceptable adaptation of a refractor of largish aperture.Many thanks, John!Mark

I also have to extend credit to Peter Wise. He also kicked me along to thinking about the possibilities; and to Mike Jones who thought of using a simple lens to compensate for the lateral color at the eyepiece. Thank you both. I hope I can get this proprietary thing off the ground, so to speak.M.

An advantage- If the components can be made fairly reliably, the positions of the achro and focus are reasonably flexible. For something other than you have to spend mega-thousands on, a working example can be made expeditiously. But I think this is falling on deaf ears. (who is the hitchin' post here???)M.

OK, I have a pile of woodchips here. I know that the big "Z" says I should make a triplet outta this stuff. But seein' that the big "Z" can't distinguish what's -REASONABLY- affordable or attainable, is there some kind of streamlining that I haven't thought of? Maybe 1 ROC that I can use on all surfaces of the correcting achromat? I have it worked down to 2 radii--- 80" and 16 point-something to get the plots above-----------******* Thank goodness for the human brain, and OSLO.M.

I'm using a positive meniscus crown BK7, 8" diameter. It has a 40"(~) ROC convex on the front, and a concave 120" on the rear. About 3/4" thick.
*******
Hey, lightning--- so far, I've made this thing by using just a grind/polish stand, a hunk of glass, and some grits. You could likely use a good piece of plate glass instead of BK7. The achro, I think I got hold of it for about $15 US; very cheap. Probably scavenge a binocular objective somewhere. So far, that's all I have in it. I did however, spend something on the BK7 chunk. If this thing -DOES- work well, I'd just as soon have a water-clear objective to keep the detractors at bay, and the rep intact! I'll try to include the basic OSLO file:- using the 'zero power' dedicated achro.
M.

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The file above should show the particulars of focal length, spacings, and radii. It could be folded with a flat for shortening up the finished scope.***** Jonesey sent me a file on a Ramsden eyepiece a while back. I have another file using a single achro from a Plossl, and one singlet, which maxed out the EDU's surface limit. Done afocally, it -APPEARS- to be correct, but I'm not 100% sure. But experiments with a std refractor, and low power eyepiece/positive singlet shows blue focusing first, or closer in across the field of view when using this singlet type eyepiece arrangement. I sized the spotplots for both eyepiece and this scope using the same scale, and ran them thru the printer on the same sheet. Just quick and dirty, they overlapped nicely, and -should- compensate each other's lateral color problem. As I said before, the singlet's distance from the eyepiece can adjust the degree of "compensation".M.

One provision: The back side of the objective has hyperboloidal correction on it. IIRC, it's around -3.75 or so... shouldn't be a problem, similar to putting overcorrection on an 8" F.7.5: probably like correcting an F/4-4.5 which is quite achieveable. This makes the LSA's green and red-blue lines stand up nice and straight, as they should be. The delta, btw, is around .050"; quite respectable for even a std achro!M.

On further thought, I'm going to clean up the back side to a sphere, and perhaps experiment with a standard achro, and a piece of negative flint and see what happens. I can always aspherize the back side later if necessary. At least I can get the back side cleaned up, and reasonably accurate.M.

I had that image of the large single front lens stuck in my head, because it reminded me of the Angenieux retrofocus. The difference being that the large objective is a negative lens producing a wide-angle short-focuser. Rich field when applied to a telescope?